transmission systems

TRANSMISSION

SYSTEMS

Transmitting large amounts of electric energy

over long distances is accomplished most

efficiently by using high-voltages.

Without transformers the widespread distribution

of electric power would be impractical. The main

purpose of a transformer is to convert AC power

at one voltage level to AC power of the same

frequency at another voltage level.

Photo courtesy ABB, www.abb.com.

High voltages are used in transmission lines to

reduce the required amount of current flow.

Power = Voltage x Current = 100 V x 100 A = 10,000 Watts

Transmission @ 10,000 volts Current reduced from 100 A to 1 A

If the voltage is raised, the current can be reduced to a small value, while still transmitting the same amount of power

Because of the reduction of current flow at high voltage, the size and cost of wiring are greatly reduced. Reducing the current also minimizes voltage drop (IR) and amount of power lost (I2R) in the lines. For a given amount of power delivered, doubling the transmission voltage cuts the electrical loses by 75 percent.

The higher the voltage, the more difficult and expensive it becomes to safely insulate between line wires, as well as from line wires to ground. For this reason, the voltages in a typical high-voltage grid system are reduced in stages as they approach the area of

final use.

Transformers make possible conversion between

high and low voltages and accordingly between

low and high currents.

Compare to the high voltage side, the low voltage side of a transformer draws more current.

UNIT

SUBSTATIONS

The place where the conversion from transmission

to distribution occurs is in a power substation. It

has transformers that step transmission voltage

levels down to distribution voltage levels.

The power needs of some

users are so great that

they are fed through

individual substations

dedicated to them. These

secondary unit substations

form the heart of an

industrial or commercial

building's electrical

system. The material and associated copyrights are proprietary to, and

used with the permission of Schneider Electric.

Unit substations receive the electric power from

the utility and step it down to the utilization

voltage level of 600 V or less for distribution

throughout the building.

Incorporates the terminations for the primary feeder cables and primary switchgear.

Steps down the primary voltage to the low-voltage utilization level.

Provides the protection and control for the low-voltage feeder circuits.

Photo courtesy Siemens, www.siemens.com.

DISTRIBUTION

SYSTEMS

Distribution systems are used to distribute power

throughout residential, commercial, and

industrial buildings.

Distribution systems that are used to

distribute power throughout large commercial

and industrial facilities can be complex.

Courtesy Siemens, www.siemens.com.

Typically the

distribution

system is

divided into

the:

Service

Entrance

Section

Feeder

Section

Branch

Circuits

Section

Distribution System - Line Diagram

The service entrance section includes conductors for delivering energy from the electricity supply system to the premises being served.

A feeder is a set of conductors that originate at a main distribution center and supply one or more secondary or branch circuit distribution centers.

The feeder section includes conductors for delivering the energy from the service equipment location to the final branch circuit overcurrent device.

The branch circuit section includes conductors for delivering the energy from the point of the final overcurrent device to the utilization equipment.

Correct selection of conductors for feeders and branch circuits must take into account ampacity, short circuit, and voltage drop requirements.

The ampacity rating of conductors in a raceway depends on: the conductor material, gauge size, and temperature rating number of current-carrying conductors in the raceway the ambient temperature

CURRENT FLOW ALWAYS GENERATES HEAT!!!

The NEC (National Electrical Code) contains tables that list the ampacity for approved types of conductor size, insulation, and operating conditions.

All conductors installed in a building must be properly protected, usually by installing them in raceways.

POWER LOSSES

The energy generated by the power plant does not

match with that distributed to the consumers.

Some percentage of the power is lost in the power

grid.

This difference is known as Transmission and

Distribution loss.

Technical losses are losses incurred by physical

properties of components in the power grid.

Power that is lost in the form of heat caused by current passing through the resistance of transmission lines and cables.

2Losses I x R

Power that is lost in transformers.

Power that is lost due to poor power factor.

Non-technical losses consist of administrative

losses, losses due to incorrect metering, non

paying customers, and theft/fraud.

Losses due to metering inaccuracies or the difference between the amount of energy actually delivered through the meters and the amount registered by the meters.

Theft of power is energy delivered to customers that is not measured by the energy meter for the customer.

SWITCHBOARDS

AND

PANELBOARDS

The NEC Electrical Code defines a switchboard as a single panel or group of assembled panels with buses, overcurrent devices, and instruments.

Typical combination service entrance and switchboard. The service entrance section has space and mounting provisions required by the local utility for metering purposes. The switchboard section controls the power and protects the distribution system through the use of switches, fuses, circuit breakers, and protective relays.

A panelboard contains a group of circuit breaker

or fuse protective devices for lighting,

convenience receptacles, or power distribution

branch circuits

They make up the part of the distribution system that provides the last centrally located protection for the final power run to the load and its control circuitry.

This material and associated copyrights are proprietary to, and used with the permission

of Schneider Electric.

Panelboards are placed in a cabinet or cutout box, which is accessible only from the front and have dead-fronts. Dead front is defined in the Code as having no exposed live parts on the operating side of the equipment.

The panelboard is usually supplied from the switchboard and further divides the power distribution system into smaller parts.

This material and associated copyrights are proprietary to, and used with the permission of Schneider Electric.

277/480-volt three-phase 4-wire panelboard

From neutral (N) to any hot line, 277 volts single-phase for fluorescent lighting can be obtained.

Across the three hot lines (A-B-C) 480 volts three-phase is present for supplying motors.

Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill

Education.

Grounding is the connection to earth, while bonding is the connection of metal parts to provide a low impedance path for fault current to aid in clearing the overcurrent protection device and to remove dangerous current from metal that is likely to become energized.

The proper grounding and bonding of electrical distribution systems in general and panelboards in particular is very important.

The main bonding jumper gives you system grounding. If a transformer is immediately upstream of the panelboard, you must bond the neutral bus or neutral conductor to the panel enclosure and to a (bare) grounding-electrode conductor.

The NEC Electrical Code requires the panelboard

cabinets to be connected to an equipment

grounding conductor. A separate equipment

grounding terminal bar must be installed and

bonded to the panelboard for the termination of

feeder and branch circuit equipment grounding

conductors The equipment grounding bus is non-insulated and mounted inside the panelboard and connects directly to the metal enclosure.

This material and associated copyrights are proprietary to, and used with the permission

of Schneider Electric.

A busbar is a common connection for two or

more circuits. Three-phase busbars are required

to have phases in sequence so that an installer

can have the same fixed phase arrangement.

Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill

Education.

Panelboards are classified as main breaker or

main lug types.

Main breaker–type panelboards have the incoming supply cables connected to the line side of a circuit breaker, which in turn feeds power to the panelboard

Photos courtesy Siemens, www.siemens.com.

Main Lug Type A main lug panelboard does not have a main circuit breaker. The incoming supply cables are connected directly to the busbars. Primary overload must be externally provided.

Normally panelboard circuit terminals are required to be labeled or to have a wiring diagram.

Photo courtesy Siemens, www.siemens.com.

MOTOR CONTROL

CENTERS (MCCs)

At times a commercial or industrial installation

will require that many motors be controlled

from a central location called a motor control

center.

A motor control center is a modular structure designed specifically for plug-in type motor control units.

Photo courtesy Rockwell Automation, www.rockwellautomation.com.

The motor control center is an assembly of

primarily motor controllers having a common

bus.

They support and house control units, a common bus for distributing power to the control units, and a network of wire trough for accommodating incoming and outgoing load and control wires.

Each unit is mounted in an individual, isolated

compartment having its own door.

MCCs are not limited to housing just motor

starters but can typically accommodate many

unit types.

Photos courtesy Rockwell Automation, www.rockwellautomation.com.

Photos courtesy Rockwell Automation, www.rockwellautomation.com.

Copyright © 2016 McGraw-Hill Education..